So the rivet were the later war pieces, you will find that the M1857 Cartridge box was also totally sewn and found to come apart at the straps, both closure and belt, which were subsequently riveted. The Pattern of 1861 had rivets on the belt loops/straps, then the Pattern 1864 had the rivet on the closing strap to reinforce the sewing (all referring to the cartridge boxes, but haversacks followed suit.)